This invention relates to a method, or an apparatus, for processing a sample by generating plasma by an electromagnetic wave. More particularly, the present invention relates to a semiconductor fabrication process, and can be suitably applied to an etching process of a metal wiring film.
Al wirings of LSls mostly employ a three-layered structure comprising a TiN cap layer/an Al--Cu alloy/a TiN barrier layer. Dry etching by a BCl.sub.3 /Cl.sub.2 gas has been widely employed for patterning this wiring. In BCl.sub.3 /Cl.sub.2 type etching, an etching rate of the Al--Cu alloy is higher than that of TiN. Therefore, there occurs the problems that side etching 305 occurs on the Al--Cu alloy layer 303 and notches 306 occur in the Al--Cu alloy layer 303 immediately below the TiN cap layer 303 as shown in FIG. 7 which shows the sectional shape of the multi-layered Al wiring according to the prior art. To accomplish an excellent processing shape, etching of the Al wiring must be carried out while controlling the formation of a side wall protective film. To cope with this problem, reduction of the notches 306 and anisotropic processing of the Al--Cu alloy layer 303 are attained by adding N.sub.2 to the BCl.sub.3 /Cl.sub.2 type gas as described, for example, in "Journal of Vacuum Science & Technology", Vol. A10, No. 4, pp. 1232-1237. The side wall protective film by this means contains BN compounds resulting from the reaction between B from BCl.sub.3 and N of the TiN layer, and the side wall protective film is more effective for controlling the shape than conventional BCl.sub.3 /Cl.sub.2 type etching.
The method described above which adds N.sub.2 to the BCl.sub.3 /Cl.sub.2 type etching gas efficiently forms the side wall protective film containing the BN compound and reduces the notches 306. However, the side wall protective film containing the BN compound involves the problem that it can not be removed easily by the ashing step and the solution treatment carried out in succession to the etching step because it has chemically strong bonds. Since the formation/deposition reaction of the BN compound occurs on the inner wall of the etching chamber, foreign matters tend to develop in the etching apparatus, and the problem occurs when this method is applied to the mass-production of the LSIs. Therefore, means for forming the side wall protective film in place of the BN compound, which is strong and firm and yet can not be easily removed after etching, becomes necessary.
Japanese Laid-Open No. 169140/1985 shows plasma etching method of aluminium or aluminium alloy under reduced pressure as a conventional art. This conventional art discloses that R.F. power of 13.45 MHz frequency is applied to an electrode on which a wafer is set, and etching gases including BCl.sub.3, Cl.sub.2 and CH4 are supplied to an etching apparatus so that fast and anisotropic etching can be achieved without damaging photoresist using low R.F power.
Requirements for a semiconductor production process utilizing plasma have become very severe, and processing accuracy, a processing shape, a selection ratio (processing speed ratio between materials), uniformity and a processing speed must be simultaneously satisfied.
A technology of forming side wall protective film has been employed in an etching process of a laminate metal wiring in order to conduct vertical processing.
A material forming a side wall protective film is mainly supplied from a photoresist. Therefore, when a selection ratio to the photoresist is increased, the supply of the material forming the side wall protective film becomes insufficient and etching in the transverse direction proceeds, so that vertical processing becomes difficult to conduct. When the material forming the side wall protective film is supplied sufficiently so as to conduct vertical processing, on the contrary, the etching speed of the photoresist must be unavoidably increased, and the selection ratio to the photoresist becomes small. In this way, processing accuracy and the processing shape have a mutual trade-off relation with the selection ratio.
On the other hand, the plasma also etches the photoresist using a plasma etching, the etched photoresist attaches to side walls of a sample, and it acts as a protective layer of the side walls. However recently there are such type of semiconductor devices appeared that the area of photoresist layer is comparatively smaller than that of entire etched surface, for example, ASIC or logic semiconductor devices. For example, a wafer is covered with an area of photoresist having 30% or smaller than 30% of entire wafer surface area. In other words, the wafer is etched with an area of exposed surface having 70% or larger than 70% of entire wafer surface area.
When etching such type of wafer, the quantity of the etched photoresist is small. Therefore the quantity of etched photoresist is not sufficient to form protective layers of side walls for controlling a shape.
The process described in Japanese Laid-Open No. 169140/1985 shows the fast and anisotropic etching method with relatively low R.F power, and the method achieves to reduce a damage of the photoresist, however, there is no suggestion about the action of protective layers of side walls.